1. Field of the Invention
The present invention relates to a diffraction grating, a light-receiving element, and an optical head and an optical recording/reproducing apparatus utilizing them.
2. Description of the Related Art
An optical recording/reproducing apparatus includes an optical head for recording information in predetermined regions of a plurality of tracks formed on, for example, a disk-shaped optical recording medium so as to extend along the circumferential direction of the optical recording medium and to repeat in the radial direction of the optical recording medium or reproducing information recorded in predetermined regions of the tracks. Optical heads include recording-only types which are used only for recording information on an optical recording medium, reproduction-only types which are used only for reproducing information, and recording/reproduction types which can be used for both of recording and reproduction of information. Therefore, apparatus carrying those types of heads constitute optical recording apparatus, optical reproducing apparatus, and optical recording/reproducing apparatus, respectively. In the present specification, all of such apparatus will be generally referred to as optical recording/reproducing apparatus.
Among optical heads used in optical recording/reproducing apparatus, a type of optical heads are widely used, which carry a light source obtained by housing a plurality of light sources emitting beams of light having different wavelengths in one package to achieve a cost reduction through a reduction in the number of components. In the case of such an optical head, diffraction gratings for splitting a light beam into a plurality of beams on an information recording surface of an optical recording medium must be provided in the optical paths of the beams having different wavelengths. Thus, the optical system of the optical head must have a space to allow the diffraction gratings to be inserted. It is therefore necessary to design the optical system of the optical head with a sufficiently long optical path length, which makes it difficult to provide the optical head in a small size.
In order to solve this problem, according to the method disclosed in Patent Document 1, two types of diffraction gratings having different grating constants are made integral with each other by forming them on top and bottom surfaces of one element to provide an optical head in a small size. Patent Document 1 also discloses a method for simplifying the adjustment of the angles of the diffraction gratings by employing a design in which the two types of diffraction gratings are disposed at predetermined angles unparallel with each other.
Patent Document 1: JP-A-2004-39109
However, the diffraction gratings disclosed in Patent Document 1 must be designed such that they will transmit the entire quantity of light having a wavelength which is not used or such that they will not diffract the light. When light having an unused wavelength is diffracted, the diffracted light constitutes stray light which will impinge upon an optical recording medium. Since the stray light enters a light-receiving element after being reflected by the optical recording medium, accurate signal detection is hindered. There is a sufficient possibility for designing a diffraction grating such that substantially no stray light is generated when there are two types of light having different wavelengths. For example, let us assume that the two types of light have wavelengths of 650 nm and 780 nm. Then, the depth of grooves in the diffraction grating is in the range from about 1200 to about 1600 nm. Examples of designs in the related art indicate that it is possible to fabricate a diffraction grating in which diffracted beams having an unused wavelength can be sufficiently suppressed when an optical head is reproduction-only type and the ratio of the quantity of a 0-order beam (main beam) diffracted to the quantity of first-order beams (sub beams) diffracted is about 6:1.
When a light beam is split by an optical head which is used also for recording, if the intensity of first-order beams is too high, recording data which have already been written can be erased by the heat of the first-order beams during a recording operation. In order to avoid this, the ratio of the quantity of a 0-order beam diffracted to the quantity of first-order beams diffracted must be set at about 18:1. However, when the ratio of the quantity of the 0-order beam diffracted to the quantity of the first-order beams diffracted is set high, a problem arises in that first-order diffracted beams having an unused wavelength cannot be eliminated completely. Specifically, when a separate diffraction grating is designed for each of beams having different wavelengths, stray light from an unused diffraction grating can impinge on a light-receiving region for first-order beams among light-receiving regions formed on a light-receiving element. Even if the quantity of the stray light incident on the light-receiving region for first-order beams is very small, the quality of the detected signal will be reduced because the light-receiving region for first-order beams has relatively high sensitivity.